Knurling for the hobbyist

 Knurling a shaft for a project can be a means of adding to the appearance of the project or for utility such as improving grip on a handle.  The nice, even knurl you saw on some other project may seem like a piece of cake, but it requires some careful operation of proper tools.  

The first thing you need to know is that different metals handle differently and will give different results with the same knurls.  Never try to speed the process.  The lathe should be running at 50 RPM or less.  Most of the time it is less.  All of my knurls are done with the scissor knurler seen on the TOOLS page of this site. The plans can be downloaded there, but are definitely not for the beginner.  I do not recommend knurling on the mini-lathe with single wheel knurlers.  The little machine is not sturdy enough for that operation.  The pressure required is just too much.

With the scissor knurler you must clamp the wheels firmly on the center of the rod being knurled.  The centering of the knurls must be watched during the operation and adjusted, if necessary, to keep them on center.  If the knurler is clamped into a tool holder, the adjustment is a simple adjustment of the cross slide.  If the rod is small and long then it must be supported with a center.  The pressure of knurling will cause flexing of a small rod.  If the rod flexes and the knurls hop off the rod it will surely be bent.

If the knurl is not very wide (3/8" or less) it can be done without moving the carriage.  Longer knurls require the carriage to be moved by the lead screw.  Done right this will create a very nice appearance for the length of the knurl.

Get your rod clamped into the chuck firmly and set the knurls over the center of the rod.  Start the lathe at a slow speed (below 50 RPM) and view the effect.  If it does not seem deep enough, tighten the knurler tension screw.  It will probably take much more pressure than you expect.  When it looks good, engage the lead screw and let it run down the piece.  While it is traveling you must keep the dust it creates off the metal.  Dust buildup in the grooves being cut will limit the depth of cut and make the knurl uneven.  I find a gentle stream of compressed air to be good for this purpose.  My shop is plumbed for 25 lb air, but I spray it at the knurl at less than the max.  Just enough to keep the dust out of the grooves.  At completion, if you find that the knurling is not deep enough you can run back the carriage and go through it again, but be sure to get the knurls clamped into the grooves exactly as they were cut before.  If the knurls are set into the existing grooves they will usually stay there for the duration of the second or sucessive runs.

When finished, the knurled area will have a surface like a rasp.  This is not something you want to grab for improved grip.  While still in the lathe, bring up the speed to 1000 RPM or more and go over the knurled area gently with a 6" mill bastard file to knock off the sharp points.  How much you want to take them down is a matter of personal preference, but do it evenly.  An uneven file down will ruin the appearance of an otherwise nice, even knurling job.

Brass is the nicest metal to knurl.  it knurls easily and the dust blows away easily.  Aluminum will try to pack the soft dust back into the grooves like mud.  Coating the knurl area with a threading lubricant often helps.  I, personally, do not recommend knurling the softer grades of aluminum.  It is just too difficult to get an even knurl.  I stick to 6061 or harder grades.  Steel cleans with air, but takes a great deal of extra pressure.  The knurler will try to ride off the work piece and must be watched carefully.  I have knurled drill rod, but it requires slow, careful work.

Nearly all knurls for grip are diamond pattern, but if you are knurling to fatten a rod prior to pressing into a hole the straight knurl is a better choice.  During pressing a diamond knurl will cut metal out of the hole enlarging it.  The straight knurl will cut straight lines in the the side walls of the hole tightening the fit and resisting turning under pressure.

I find that 3/4 X 3/8 knurl sets are usually available for 6 to 7 dollars per set (try littlemachineshop.com or Enco manufacturing at use-enco.com).  I have the diamond patterns in fine, medium and coarse and a straight set.  These four patterns do everything that comes up nicely.

 

The knurler is clamped on to a hammer handle traveling down the length under lead screw power.   A locking knob is being knurled here.  Since the knob is not wider than the knurls, there is no travel.  It is just knurled to correct depth and stopped.
 

These tool handles have been knurled to improve grip.    This is a die holder that allows threading rods on the lathe.  The knurl allows a firm grip while in use.

The knobs on this scriber have been knurled for both appearance and grip.  The knobs are diamond knurl, but the fine adjust roller was straight knurled since it will be rolled sideways by fingers or thumbs.